OO-1873: replace the listing of booking by a native query optimized for our standard setup, replace the .hbm.xml with annotations, add more fetching overall

OO-1606: close asap the database connection in the QTIPreWarm executor, remove some unused spring dependencies as Eclipse Mars detect them

OO-1483: implement a prewarm infrastructure to warm-up openolat caches, implement it for velocity templates, display names of users and QTI 2.1 resources